This invention relates to the bonding of a chip to a substrate and, more particularly, to a method and fixture for use in such a bonding process to provide increased throughput.
At the present time, semiconductor laser chips are mounted to a silicon optical bench (or substrate) one at a time. In present practice, the substrates are predeposited with solder and a pick and place machine uses a vacuum collect to pick up a single substrate from a substrate cassette and place it on a lower vacuum chuck. The chip is then picked up by the upper vacuum chuck and held on top of the substrate. Heat is then applied to the substrate and chip through the chucks to melt the solder and bond the chip to the substrate. A number of problems are associated with this present practice.
The present practice is time consuming. Wafer level assembly or multiple substrate assembly is impossible because the substrate will not withstand multiple solder reflows, which deteriorate the solder composition. In addition, the time necessary for the bonding, which includes tacking and heating/cooling, is twice as long as for just tacking.
The present practice also results in poor solder joint quality. Thus, since the solder is reflowed in an open environment, even with a gas purge oxidation is still unavoidable. Further, the final composition of solder joints vary on a single substrate with multiple joints because of the nature of the sequential bonding. Also, the process has to be changed from time to time when a different substrate with different joints is used.
It would therefore be desirable to be able to perform wafer-level assembly or multiple substrate assembly, without the disadvantages described above.